The present invention relates in general to valves which control the passage of fluid between two or more locations. More specifically the present invention relates to a three-way, solenoid valve which is used to control the flow of fluid between three locations in a fuel delivery system.
One means of controlling the precise delivery of a high pressure fluid is to use a solenoid valve where the armature of the solenoid either controls or is configured similar to the shape of a spool valve. Either one or two solenoids may be used to control movement of the spool valve between a valve-closed position and a valve-opened position. Typically the spool is configured with enlarged ends and an intermediate blocking portion with reduced diameter portions between each end and the intermediate blocking portion. The ends of the spool valve may be utilized as part of the sealing within the valve housing. The reduced diameter portions between the enlarged ends and the intermediate blocking portion provide for the flow of fluid over and around the spool valve. In the valve-opened condition, a flow path is established for pressurized fluid to enter through an inlet passageway and exit through an outlet passageway. The speed of the solenoid armature and thus the speed of the spool valve moving from a closed to an opened which is delivered and the preciseness of the opening and closing times.
When a solenoid valve of the type described is used in conjunction with a fuel injector, for example, there will be a need to permit the backflow of fuel to an open drain port in the solenoid valve which will return this fuel to the fuel tank or other fuel reservoir. By allowing some of the fuel to drain from the injector, this relieves the fluid pressure on the metering plunger of the injector. By relieving this fluid pressure the metering plunger is allowed to return to its normal position for the start of another injection cycle.
A typical solenoid valve for this application would include an inlet port or aperture, an outlet port, and a separate drain port, all fabricated into a valve housing and internally flow coupled. Typically these three ports or passageways are arranged to intersect a common central core opening extending through the valve housing. This central core which is generally cylindrical is constructed and arranged to receive the spool valve. In the opened condition, pressurized fuel is able to travel from the inlet to the outlet over and around the spool valve. In this configuration, the drain passageway is closed. After the charge of fuel has been delivered, the solenoid valve shifts to its closed condition which prevents the introduction of any more pressurized fuel through the inlet port. In the closed condition, flow communication between the outlet port and the drain port is permitted and this allows the fuel in the injector to drain back so as to prepare the injector for the next charge of fuel which it will receive.
In this typical type of solenoid valve there are two current concerns which could constitute significant problems depending on their extent or magnitude. The first concern relates to a transition loss which can occur during movement of the spool valve between its closed and opened conditions. When the spool valve moves from a closed condition to an opened condition there is an opportunity for a small amount of the entering high pressure fluid to escape to the drain port rather than all of the entering fluid going to the outlet port. Thus, during the "transition" of the spool valve from a closed to opened condition, there is some fluid "loss" to the drain.
The second concern relates to cavitation which is caused in part by the speed of the spool valve as it moves through the valve body. Transition loss and cavitation significantly affect system performance and affect the development of high performance solenoid valves. It is therefore important to design a three-way solenoid valve such that transition loss and cavitation are reduced.
It is known that the larger the drain orifice area, the more significant the transition loss and cavitation for a given spool valve speed. If the sole issue is the diameter size of the drain orifice relative to the opening speed of the solenoid valve, a smaller drain orifice could be considered. Experimental work with different orifice sizes for the drain passageway has confirmed that a smaller drain orifice does improve valve performance. This experimental work compared a drain orifice with a diameter size of 0.06 inches (1.52 mm) as compared to a drain orifice diameter size of 0.04 inches (1.02 mm). A second concern then becomes the draining duration from the injector as the length of the draining duration is directly proportional to the reciprocal of the drain orifice diameter. It has been determined that the end of the draining duration can be allowed to go up to 720 degrees of crank angle after fuel injection. This indicates that significantly smaller drain orifice diameters can be used, such as diameters in the 0.01 inch (0.25 mm) to 0.02 inch (0.51 mm) range.
While this "theory" is analytically sound, there are other manufacturing realities to consider, such as the cost of the solenoid valve if a drilled drain passageway in the 0.01 inch (0.25 mm) diameter range is required. A larger drain orifice would certainly reduce the valve cost, but this would bring back into play the associated concerns of cavitation and transition loss. Therefore, a solution that balances these competing interests is needed.
The present invention provides a unique and obvious solution to this problem by creating a variable drain orifice which is based on a larger drilled bore for the drain passageway and orifice but which uses the spool geometry to reduce cavitation and transition loss.
Over the years, a variety of solenoid valve designs have been contemplated, several of which have become the subject of patent applications and issued patents. Below is a listing of six issued United States patents which are believed to provide a representative sampling of these earlier designs. Also listed is a pending U.S. patent application which discloses a three-way valve having various flow channels formed between the valve member and the valve housing.
______________________________________ U.S. Pat. No./Ser. No. INVENTOR ISSUE DATE ______________________________________ 3,587,156 Sorenson Jun. 28, 1971 3,921,660 Kowalski Nov. 25, 1975 4,582,294 Fargo Apr. 15, 1986 4,598,736 Chorkey Jul. 8, 1986 4,726,493 Wallace et al. Feb. 23, 1988 4,932,439 McAuliffe, Jr. June 12, 1990 5,396,926 Pataki, et al. Mar. 14, 1995 ______________________________________